1. Field of the Invention
The present invention relates to a vehicle surroundings monitoring apparatus which is capable of detecting objects such as vehicles lying in the surroundings of a vehicle of concern, and which is usable with such a device as an intervehicle (vehicle to vehicle) distance warning device, an intervehicle distance control device, a backside warning device or the like, for generating a warning or controlling the vehicle of concern based on the distance of the vehicle of concern to other vehicles lying around the vehicle of concern. More specifically, the present invention relates to such a vehicle surroundings monitoring apparatus which can exhibit the utmost effect particularly in the case of using an optical radar or a laser radar with high sensitivity and a wide horizontal viewing angle.
2. Description of the Related Art
In the past, for a preventive safety device for a vehicle, there has been proposed an intervehicle distance warning apparatus which is mounted on a vehicle of concern (hereinafter simply referred to as a subject vehicle) for measuring the distances from the subject vehicle to other vehicles, particularly to a vehicle running ahead thereof (hereinafter simply referred to as an intervehicle distance) and generate a warning to the operator of the subject vehicle when the intervehicle distance becomes dangerous, i.e., below a prescribed distance. Also, there has been proposed an intervehicle distance control apparatus which adjusts the running speed of a subject vehicle in an automatic manner so as to prevent the intervehicle distance to a vehicle running ahead thereof from becoming shorter than a prescribed value. In these apparatuses, a vehicle surroundings monitoring apparatus using a scanning type laser radar has often been employed for measuring the intervehicle distance and the relative speed of the subject vehicle to the speed of a preceding vehicle running ahead thereof, which is the rate of change of the intervehicle distance.
The scanning type laser radar scanningly irradiates a transmission wave or laser beam over a prescribed angle around the subject vehicle, detects the returned wave or beam reflected by surrounding objects, and measures the distance from the subject vehicle to each object in each radiation direction based on a time span between a transmission time point, at which the transmission wave or beam was irradiated, and a detection time point, at which the reflected wave or beam was received. Therefore, in order to detect surrounding objects, it is necessary to calculate the relative positions and the relative speeds of these objects with respect to the subject vehicle after the objects such as vehicles and obstacles are reconstructed in a measurement space on the basis of the distance data measured in the respective radiation directions. Such a kind of conventional apparatuses are described, for example, in Japanese Patent Application Laid-Open No. 5-180933 and Japanese Patent Application Laid-Open No. 7-318652.
With the apparatus described in the Japanese Patent Application Laid-Open No. 5-180933, an obstacle is first recognized as a set of blocks each of which has a predetermined area based on the detection results of the reflected wave, and the position of the center of gravity thereof is then detected. Subsequently, the position at which the center of gravity of the obstacle is expected to be detected at the next scanning is estimated, and when the center of gravity of the obstacle recognized as the set of blocks is actually detected at the estimated position at the next scanning, it is determined that both of the obstacles actually detected and estimated are the same object. Thereafter, the relative speed of the vehicle to the obstacle is calculated from the rate or amount of change and the difference in time of the positional data of the center of gravity of the obstacles which have been determined to be the same object.
However, since in such a kind of apparatus, an obstacle is recognized as a set of blocks each having a prescribed area, a large amount of information is required to define one obstacle, thus complicating the processing of data. Therefore, the load of a processing device such as a computer has increased. Moreover, since roadside objects such as guardrails are recognized as obstacles, the load of the processing device has further increased in the case where these roadside object exist. Therefore, with these apparatuses, it has been impossible to improve the processing speed and accuracy in the obstacle recognition to any satisfactory extent.
In view of these circumstances, the Japanese Patent Application Laid-Open No. 7-318652 proposes to achieve the detection of surrounding objects according to the following procedure.    1) An obstacle around a subject vehicle is recognized as points.    2) Adjoining ones of the recognized points are combined or integrated with each other into sets of points.    3) Among the integrated sets of points, those sets each of which has a longitudinal length less than a prescribed value in the longitudinal direction of the subject vehicle are recognized as line segments each having a length only in the widthwise direction of the subject vehicle.    4) That position of each of the line segments recognized at a past scanning which is to be expected at the current scanning is estimated, and if the line segment currently recognized is located at the estimated position, it is determined that the past recognized line segment is the same as the current recognized line segment.    5) The relative speed between the subject vehicle and the obstacle is calculated from the positional data of the line segments which are determined to be the same one.
In this manner, the amount of information necessary to define one obstacle is reduced so as to prevent roadside objects such as guardrails from being recognized as obstacles.
However, the conventional apparatuses as described above are based on the following specifications as their premise.    a) A horizontal viewing angle is set to about 16 degrees in order to detect vehicles or obstacles present on a lane on which the subject vehicle is running.    b) Major objects to be detected are highly reflective elements such as reflectors mounted on the rear end of a vehicle, delineators installed on the shoulders of a road, or the like.
Therefore, in the case where a scanning type laser radar is used which is so sensitive as to be able to detect the body of a vehicle and has so wide a horizontal viewing angle as to able to detect vehicles running on lanes adjacent the lane on which the subject vehicle is running in order to improve the detection rate or efficiency or detect an interrupting vehicle, there will arise the following problems with the conventional apparatus, for example, as set forth in the Japanese Patent Application Laid-Open No. 7-318652, thus making such an application difficult.    A) Since the method for determining that the line segment recognized at a past scanning is the same as the line segment recognized at the current scanning is complicated, the number of line segments to be detected increases as the sensitivity and the horizontal viewing angle increase, so the number of combinations of the line segments for determining the same line segment increases accordingly, thereby requiring quite a lot of processing time. That is, the increasing number of detected line segments makes it difficult to perform the recognition processing at a prescribed period or interval.    B) Also, in actually constructing such an apparatus, it is necessary to estimate the maximum amount of processing at the time when the number of segments is increased, in order to determine the capacity of processing required of the apparatus, but with the conventional apparatus, all the possible combinations of line segments must be conceived and examined, and hence it becomes extremely difficult to estimate the maximum amount of processing. Therefore, it is impossible to ensure that the recognition processing can be done at the prescribed cycle under all situations.